1. Field of the Invention
The present invention relates to a semiconductor device including a power element and a control semiconductor element for controlling the power element and a method of fabricating the semiconductor device.
2. Description of the Background Art
FIG. 8 is a cross-sectional view of a conventional multichip power semiconductor device including a power element and a control semiconductor element for controlling the power element, and FIG. 9 is a plan view of the power semiconductor device of FIG. 8. The power semiconductor device shown in FIGS. 8 and 9 will be described hereinafter with process steps for fabricating the same.
A power element 3, a control semiconductor element 2 and a passive element 8 are soldered with solder 6 in position onto a lead frame 1 which is heated. The control semiconductor element 2 controls the power element 3. The passive element 8, formed by capacitors and chip resistors, enhances the characteristics of the power semiconductor device, for example, suppresses oscillation.
The power element 3 and control semiconductor element 2 are wire bonded to the lead frame 1 with aluminum wires 5 and gold wires 4. Transfer molding is carried out by using packaging resin 7 to integrally package the power element 3, control semiconductor element 2 and passive element 8. The lead frame 1 is cut along the dashed-and-dotted line of FIGS. 8 and 9 to remove a tie bar 11. This permits the lead frame 1 to be separated into leads. A hole is previously formed in a predetermined portion of the lead on which the power element 3 is mounted. A hole 100 is formed in the packaging resin 7 in corresponding relation to the hole in the lead after the transfer molding. Thus, the threaded hole 100 is formed.
The reverse face of the power element 3 serves as a collector. The reverse face of the lead which contacts the collector is exposed to radiate heat generated by the power element 3.
After cutting off the tie bar 11, an electric signal is applied to the respective leads to inspect the power semiconductor device. Since the reverse face of the power element 3 serves as the collector as above described, the lead on which the power element 3 is mounted is at a collector potential. For insulation from the collector of the power element 3, the control semiconductor element 2 is mounted on a concave portion (not shown) referred to as an island which is provided in one of different leads from the lead on which the power element 3 is mounted.
Description will be given of a process for mounting the power semiconductor device thus fabricated on a conductive external mounting base plate 15 with reference to FIG. 10. It is necessary in this case to insulate the external mounting base plate 15 from the reverse face of the lead exposed for heat radiation. One of the insulating methods is to provide the power semiconductor device of a full-mold type. The full-mold type power semiconductor device enables complete insulation. However, since the exposed portion of the lead is also molded, the heat radiation deteriorates, resulting in restriction of the power of the power semiconductor device.
There is provided a power semiconductor device which has an uneven fin mounted on the exposed reverse face of the lead frame 1 to improve the heat radiation of the semiconductor device. For mounting the semiconductor device on the external mounting base plate 15, it is necessary to provide an insulating sheet 12 made of material that contains silicon between the power semiconductor device and external mounting base plate 15 as shown in FIG. 10.
The conventional power semiconductor device thus constructed has the following problems: firstly, the power and control semiconductor elements 3 and 2, which are mounted on different leads, are located at a distance so that it is inconvenient to put them in a single package. Another problem is that, since the respective elements are electrically connected through the tie bar 11 before the lead frame 1 is separated into the leads, the inspection of the semiconductor device cannot be performed until the tie bar 11 is cut off after packaging. The conventional power semiconductor device is also disadvantageous in that it is necessary to mount the power semiconductor device on the conductive external mounting base plate 15, troublesomely, with the insulating sheet 12 fitted therebetween. Furthermore, the lead frame 1, which is required to support the weight of parts mounted thereon until the packaging resin 7 seals the lead frame 1, must have rigidity for transfer molding. Thus the leads of the lead frame 1 cannot be made thin. This results in difficulty of increasing a wiring density between the semiconductor elements, if in plurality, mounted on the lead frame 1.